Function: maintain fluid/electrolyte, acid/base balance, excrete metabolites, xenobiotics, regulate blood pressure; secrete hormones. The function is evaluated by bun (blood, urea, nitrogen) and creatinine levels; look at the level of protein metabolites. It has very high blood flow; any toxin in the circulation will flow through the kidney multiple times at a rapid rate. 21% of the cardiac output (1. 2 l/min) goes through the kidney. The glomerular filtration rate and the renal perfusion rate are maintained at a constant level in the kidney due to safety mechanisms. Excretion risks: high concentrations in the nephron. Intratubular ph may vary and lead to precipitation of compounds. Each kidney has ~1 million nephrons; they can repair but cannot regenerate. The urine ends up in collecting ducts where it will eventually be taken into the bladder. Resistance: there is a large functional reserve in the kidney; there are spare nephrons that allow for some damage, ability to hypertrophy.